FIG. 1 illustrates a typical hybrid-fiber-coax (HFC) cable system architecture. A headend 100 communicates with hub 105. Hub 105 comprises a cable modem termination system and switching/routing components. Hub 105 communicates with a node 110. Node 110 provides an interface between the fiber-based transport medium of the cable network (between the headend 100 and upstream side of node 110) and the coax-based medium (between the downstream side of node 110 and the video termination devices 115i-115n). As will be appreciated by those skilled in the art, FIG. 1 is a simplified schematic of a typical cable network architecture.
HFC cable systems have historically been “unswitched” distribution systems of video content. That is, each video termination device 115i-115n connected to node 110 received all of the video channels broadcast by headend 100. Because many of these channels will not be actively viewed by a subscriber, the unswitched distribution of content is wasteful of bandwidth.
In a switched digital video (SDV) system, unwatched channels may be deleted from the broadcast stream. In order to accomplish this, the viewing state of each video termination device 115i-115n is monitored over a two-way channel between the video termination device and the distribution hub connected to it. The video termination device sends a channel request signal back to the distribution hub. If a channel is not currently being transmitted on the coaxial line, the distribution hub may allocate a new QAM channel and, if allocated, transmits the new channel to the coaxial cable via the fiber optic node.
FIG. 2 illustrates the logical components of an SDV system. In an SDV system, a cable headend 205 comprises an SDV manager 210 and an SDV server 215, a hub 220 comprising an edge-QAM (quadrature amplitude modulation) modulator 225, and a video termination device 235 comprising an SDV client 240. A node 230 provides an interface between the fiber portion of the HFN and the coax portion of the network.
In a typical headend, content is received from multiple sources, including satellite, terrestrial over-the-air broadcast, fiber transport, storage media, and IP data networks. The receiving equipment for these sources uses various physical connections and interfaces.
As in unswitched systems, the QAM modulator 225 enables the transmission of a multiplex of digital streams via an RF carrier in an HFC spectrum channel. The digital streams may be composed of strictly MPEG transport packets or may contain IP packets as do QAM streams used for DOCSIS cable modem termination systems (CMTSs). The SDV system uses the QAM modulator 225 to request (join) and terminate (leave) IP multicasts and to transmit programs as MPEG transport packets in RF.
The SDV manager 210 governs access to content and network resources and allows sharing of those resources by various applications. The SDV server 215 uses a session setup protocol (SSP) to request from the SDV manager 210 shell sessions on a QAM signal feeding a given service group. The SDV manager 210 identifies available bandwidth on the service group QAM signal and provides shell-session space to the SDV server 215, thereby reserving bandwidth on the QAM modulator 225 for exclusive use by the SDV server 215. In the reply from the SDV manager 210, the SDV server 215 is given the control IP address of the QAM modulator 225 so that the SDV server 215 may directly control session bindings. Prior to granting QAM shell-session space (and thereby bandwidth) to the SDV server 215, the SDV manager 210 sets up the actual shell sessions on the selected QAM modulator 225 in order to prepare it for binding requests from the SDV server 215. The QAM modulator 225 is not told what server may request these bindings since they may come from a primary or a backup SDV server.
Since the SDV manager 210 is the master bandwidth controller in the system, it may need to recover bandwidth previously assigned to the SDV server 215. It may do so by sending a bandwidth reclamation request to the SDV server 215 for a specified service group. Upon receipt of such a request, the SDV server 215 initiates a QAM session teardown request for sufficient shell-session bandwidth to cover the reclamation.
The SDV server 215 is part session manager in that it directly receives and processes channel change requests received from the SDV client 240 that resides on video termination device 235. It is also part resource manager in that, for its allocation of QAM shell sessions, it can bind and thus assign those to real programs for transmission to the service groups.
The SDV server 215 receives channel change requests for switched content from the SDV client 240 to bind that content to a session on QAM modulator 225 feeding a service group associated with video termination device 235. The SDV server 215 responds to video termination device 235 with the frequency and program number where that content may be found. The SDV server 215 also fields channel change request messages for non-SDV broadcast channels in order to gather anonymous usage statistics and understand activity.
The SDV client 240 is a software component that is integrated in the resident application (navigation guide) of an SDV-enabled video termination device 235. The operation of the SDV client 240 is transparent to the user. The SDV client 240 enables the video termination device 235 to communicate to the SDV server 215 using an interface such as the SDV Channel Change Message Interface Specification (CCM).
The configuration of the SDV system is dynamic. The state of the SDV system at a particular point in time depends on a number of factors, including the program selection of members of each service group, the demands of each service group on the system resources, and the health of the system resources. The SDV server 215 is tasked with maintaining the state of the SDV system. A failure of the SDV system to respond to changes in the factors that affect the system configuration may result in subscriber dissatisfaction, inefficient operation and even catastrophic system failure.